Schottky diodes are named after their inventor, Dr. Schottky, short for Schottky barrier diode (abbreviated SBD). SBD is not fabricated by the principle of forming a PN junction by contacting a P-type semiconductor with an N-type semiconductor, but by a metal-semiconductor junction based on metal-to-semiconductor contact principle. Therefore, SBD is also called a metal-semiconductor diode or a surface barrier diode, which is a hot carrier diode.

This video will introduce the Schottky diode, also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal.

Principle

The Schottky diode is a metal-semiconductor device made of a noble metal (gold, silver, aluminum, platinum, etc.) as a positive electrode A, an the N-type semiconductor as a negative electrode B, and a barrier having a rectifying characteristic formed on the contact surface thereof. Since a large amount of electrons are present in the N-type semiconductor, only a very small amount of free electrons are present in the noble metal, resulting in electrons diffuse from the high concentration B to the low concentration A. And there is no hole in A, therefore, there is no diffusion movement of the holes from A to B. As the electron continuously diffuses from the B to the A, the electron concentration on the surface of the B is gradually reduced, destroying the surface electric neutrality, and the potential barrier is formed, and the electric field direction thereof is from B to A. However, under the action of this electric field, the electrons in A will also produce drift motion from A to B, which weakens the electric field formed by diffusion motion. When a certain space charge region is established, the electron drift motion caused by electric field and the electron diffusion motion caused by different concentrations reach a relative balance, that is the forming principle of SBD.

Electronic Symbol

The internal circuit structure of a typical Schottky diode is based on an N-type semiconductor, on which an N- epitaxial layer using arsenic as a dopant is formed. The anode is made of a material such as molybdenum or aluminum to form a barrier layer, and SiO2 is used to eliminate the electric field in the edge region and increase the withstand voltage of the tube. The N-type substrate has a small on-state resistance, and its doping concentration is 100 times higher than that of the H-layer. An N+ cathode layer is formed under the substrate to reduce the contact resistance of the cathode. By adjusting the structural parameters, a Schottky barrier is formed between the N-type substrate and the anode metal. When a forward bias is applied to both ends of the Schottky barrier (anode metal is connected to the positive electrode of the power supply, and the N-type substrate is connected to the negative electrode), the Schottky barrier layer is narrowed, and the internal resistance becomes small; When a reverse bias is applied across the Schottky barrier, the Schottky barrier layer becomes wider and its internal resistance becomes larger.

In short, the structural principle of the Schottky diode is very different from that of the PN junction diodes. The PN-junction type is usually called a junction diode, and the metal-semiconductor contact diode is called a Schottky diode. In addition, aluminum-silicon Schottky diodes fabricated using a silicon planar process have also been introduced, which not only saves precious metals, significantly reduces costs, but also improves parameter consistency.

Advantages

SBD has the advantages of high switching frequency, low forward voltage, and low reverse breakdown voltage, mostly less than 60V, and the maximum is only about 100V, which limits its application range. For example, in the switching power supply (SMPS) and power factor correction (PFC) circuits, the freewheeling diode of the power switching device, the transformer secondary etc., those are all require a high-frequency rectifier diode of 100V or more. And the RCD snubber circuit uses a high-speed diode of 600V to 1.2kV, only a fast recovery epitaxial diode (FRED) or an ultra fast recovery diode (UFRD) can meet the requirement. The reverse recovery time of UFRD is also above 20 ns, which cannot meet the needs of SMPS of 1 MHz to 3 MHz in areas such as space stations. Even with a SMPS of 100 kHz, since the conduction loss and switching loss of the UFRD are large, the case temperature is high, and a large heat sink is required, so that the volume and weight of the SMPS are increased, which is not compatible with miniaturization trend. Therefore, the development of high-pressure SBD above 100V has always been a research topic and a hot spot of concern. In recent years, SBD has made breakthroughs. High-voltage SBDs of 150V and 200V have been launched, what’s more, SBDs with more than 1kV made of new materials have also been successfully developed.

Disadvantages

The biggest disadvantage of Schottky diodes is their low reverse bias and large reverse leakage current. For Schottky diodes made of silicon and metal, the reverse bias voltage is rated up to 50V. The reverse leakage current value is in a positive temperature characteristic, which is easy to become larger as the temperature rises. So it is necessary to keep an eye on the thermal runaway. To avoid the above problems, the reverse bias voltage of the Schottky diode in actual application will be much smaller than its rated value. However, the technology of Schottky diodes has also advanced, with reverse bias ratings up to 200V.

Structure

The structure and materials of the new high-voltage SBD are different from those of traditional SBD. A conventional SBD is formed by contacting a metal with a semiconductor. The metal material may be aluminum, gold, molybdenum, nickel, titanium etc., and the semiconductor is usually silicon (Si) or gallium arsenide (GaAs). Since the electron mobility is larger than the hole mobility, in order to obtain good frequency characteristics, an N-type semiconductor material is selected as the substrate. In addition, reducing the junction capacitance of the SBD and increasing the reverse breakdown voltage without making large series resistance, a high-resistance N- thin layer is usually added on the N+ substrate.

Schottky Diode Structure Diagram

If the Schottky diode pin material is made of high-purity oxygen-free copper, which will improve the conductivity and thickness. It also has better conductivity and will not heat up for a long time. While the ordinary copper wire will be easily oxidized and soldered, resulting in low performance and easy breakage.

Schottky Diode Diagram

It is well known that there are a large amount of conductive electrons inside a metal conductor. When the metal is in contact with the semiconductor (the distance between them is only an atom), the Fermi level of the metal is lower than the Fermi level of the semiconductor, and the electron density is smaller than the electron density of the semiconductor conduction band compared with the semiconductor conduction band inside the metal. Therefore, after they connect, electrons diffuse from the semiconductor to the metal, causing the metal to carry a negative charge and the semiconductor is positively charged. Because metal is the ideal conductor, the negative charge is only distributed within a thin layer of atomic size. In the case of an N-type semiconductor, impurity atoms that lose electrons become positive ions, and are distributed in a large thick layer. As a result of the diffusion of electrons from the semiconductor to the metal, a space charge region, a self-built electric field and a barrier are formed, and the depletion layer is only on the side of the N-type semiconductor (the barrier regions all in the semiconductor side). The direction of the self-built electric field in the barrier region is directed to the metal by the N-type region. As the thermal electron emission increases from the built field, the drift current opposite to the direction of the diffusion current increases, eventually reaching dynamic equilibrium, forming a contact barrier between the metal and the semiconductor, and this is the Schottky barrier.

When the applied voltage is zero, the diffusion current of the electron is equal to the reverse drift current, achieving dynamic balance. When the forward bias is applied (that is, the metal is applied with a positive voltage and the semiconductor is applied with a negative voltage), the self-built field is weakened, and the semiconductor side barrier is lowered, so that a forward current from the metal to the semiconductor is formed. When reverse bias is applied, the self-built field is enhanced and the barrier height is increased to form a small reverse current from semiconductor to metal. Therefore, SBD, like the PN junction diode, is a nonlinear device with unidirectional conductivity.

Package

Schottky diodes are available in both leaded and surface mount (SMD) packages, and Schottky diodes with surface mounts are available in a variety of packages, including single-tube, double-tube, and triple-tube versions. Schottky diodes in leaded packages are commonly used as high frequency, high current rectifier diodes, freewheeling diodes or protection diodes. It is available in single-tube and dual-diode packages. In addition, Schottky has three types of pinout for the tube, that is, a common cathode (the cathodes is connected), a common anode (the anodes is connected), and a series (the anode of one diode is connected to the cathode of the other diode).

Technique Parameters

1) On-voltage drop VF: VF is the voltage drop across the Schottky diode when the Schottky diode is in forward conduction. Select it needed to pay more attention to the VF.

2) Reverse saturation leakage current IR: IR refers to the current flowing through the tube when a reverse voltage is applied across it. Because the Schottky diode has a large reverse leakage current, smaller IR is required when chose one.

3) Rated current IF: IF refers to the average current value calculated by the allowable temperature rise when the diode is operated for a long period of time.

4) Surge current IFSM: The forward current at the moment when power on. It is not a normal current, but an instantaneous current, which is quite large.

5) Reverse peak voltage VRM: Even if there is no reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later. VRM refers to the maximum reverse voltage that can be applied to avoid breakdown.

6) DC reverse voltage VR: The above-mentioned VRM is a repeatedly applied peak voltage, and VR is a value when a DC voltage is continuously applied. For DC circuits, the maximum DC reverse voltage is important to determine the allowable and upper limits.

7) Operating frequency FM: Due to the junction capacitance of the PN junction, when the operating frequency exceeds a certain value, its unidirectional conductivity will deteriorate. Schottky diodes have high FM values up to 100 GHz.

8) Reverse recovery time Trr: When the operating voltage changes from a forward voltage to a reverse voltage, the ideal operation of the diode is that the current can be instantaneously turned off. In fact, it usually takes a little delay. The amount that determines the current cutoff delay is the reverse recovery time. In other words, when the Schottky diode is suddenly reversed by conduction, the reverse current is greatly attenuated to a time required to approach IR. It directly affects the switching speed of the diode, but does not mean that this value must be smaller. And this indicator is important when the high-power switch is operating in the high-frequency switch state.

Features

1) Since the Schottky barrier height is lower than the PN junction barrier height, its forward conduction threshold voltage and forward voltage drop are both lower (about 0.2V lower) than the PN junction diode.

2) Since SBD is a majority carrier conductive device, there are no minority carrier lifetime and reverse recovery problems. The reverse recovery time of the SBD is only the charge and discharge time of the Schottky barrier capacitor, which is completely different from the reverse recovery time of the PN junction diode. Since the reverse recovery charge of the SBD is very small, the switching speed is very fast and the switching loss is also very small, which is especially suitable for high frequency applications. However, the reverse barrier of the SBD is thin and breakdown is more likely to occur on the surface thereof, the reverse breakdown voltage is relatively low. Because the SBD is more susceptible to thermal breakdown than the PN junction diode, the reverse leakage current is larger than the PN junction diode.

Schottky Barrier Diode (SBD)

The biggest bright feature of a Schottky diode is unidirectional conduction, which means that current can only pass in a single direction. Because of this characteristic, it is often used as a switching element to control current.

Application

Schottky diodes are widely used in various self-control circuits, communication circuits, instrumentation circuits, home computer circuits and televisions because of their fast switching speed, long life, no contact, small size and high reliability. They are also used in DVD players, video recorders and other circuits.

The structure and characteristics of the SBD make it suitable for high-frequency rectification in low-voltage, high-current output situations, and for detection and mixing at very high frequencies (such as X-band, C-band, S-band, and Ku-band). Used as a clamp in high speed logic circuits. SBD is also commonly used in ICs.

Schottky diodes are low power, ultra high speed semiconductor devices. The most notable feature is that the reverse recovery time is extremely short (can be as small as a few nanoseconds), and the forward voltage drop is only about 0.4V. It is widely used as high frequency, low voltage, high current rectifier diode, freewheeling diode, protection diode, and also used as a rectifier diode and small signal detection diode in circuits such as microwave communication. It is more common in communication power supplies and inverters.

A typical application of Shockley diode is in the switching circuit of the bipolar transistor BJT. Clamp by connecting a Shockley diode on the BJT, so that the transistor seems in a off state when it is in the on state to improve the switching speed of the transistor. This method is used in TTL internal circuits of typical digital ICs such as 74LS, 74ALS, and 74AS.

The biggest feature of Schottky diodes is that the forward voltage drop VF is relatively small. In the case of the same current, its forward voltage drop is much smaller. In addition, its recovery time is short. It also has some disadvantages: low withstand voltage and larger leakage current. It is necessary to consider them when select a Schottky diode.

Detection

Schottky diode is widely used in switching power supplies, inverters, drivers and other circuits as high-frequency, low-voltage, high-current rectifier diodes, freewheeling diodes, and protection diodes. Its main faults are open circuit, short circuit and unstable voltage regulation.

When using a digital multimeter to test the Schottky diode, the red pen is connected to the negative pole of the Schottky diode and the black pen is connecting the negative pole of the diode. The resistance value measured at this time is the forward conduction resistance of the Schottky diode.

Note

The difference between the fast recovery diode and the Schottky diode?

The structural principle is different. The Schottky diode is a combination of a noble metal and an n-type semiconductor, while the fast recovery diode is a common pn junction with a thin base.

The Schottky rectifier uses only one kind of carrier (electron) to transport the charge, and there is no excess of minority carrier accumulation outside the barrier. Therefore, there is no charge storage problem, which makes the switching characteristics visible. Its reverse recovery time can be shortened to less than 10ns. However, its reverse withstand voltage is low, generally not exceeding 100V. Therefore, it is suitable to work under low voltage and high current conditions. With its low voltage drop, it can improve the efficiency of low voltage, high current rectification (or freewheeling) circuits.

Schottky Diodes

The fast recovery diode refers to a diode with a short reverse recovery time (below 5us). The process is mostly gold-doped. Most are PN junction structure, and some adopt an improved PIN structure. Its forward voltage drop is higher than that of ordinary diodes (1-2V), and the reverse withstand voltage is more than 1200V. In terms of performance, it can be divided into two levels: fast recovery and ultra fast recovery. The former reverse recovery time is hundreds of nanoseconds or longer, while the latter is below 100 nanoseconds.

Schottky diodes, having advantages of low forward voltage drop (0.4~1.0V) and short reverse recovery time (0~10 nanoseconds), with the disadvantages of large reverse leakage current, and low withstand voltage, generally lower than 150V, are mostly used in low voltage applications.

The reverse breakdown voltage of the Schottky diode is mostly not higher than 60V, and the maximum is only about 50V, which is not suitable for high voltage circumstance. The reverse peak of the fast recovery diode can be several hundred to several thousand volts, for example, as high-frequency rectifier diodes in the switching power supply transformer circuit.

The recovery time of Schottky diode is about one hundred times smaller than that of the fast recovery diodes, generally about several nanoseconds.

The advantages of the former are low power consumption, high current, and ultra high speed. Fast recovery diodes use gold doping, simple diffusion and other processes to obtain higher switching speeds and higher breakdown voltages. Currently, fast recovery diodes are mainly used in inverter power supplies as rectifying components.

Schottky diodes: It is a diode with a "metal semiconductor junction", and its forward starting voltage is low. In addition to the material, the metal layer may also be made of gold, molybdenum, nickel, titanium or the like; using silicon or gallium arsenide, most semiconductor materials are N-type. This device is electrically conductive by majority carriers, so its reverse saturation current is much larger than that of diodes based on PN junction.

Because the minority carriers memory effect in Schottky diodes is very small, its frequency response is only limited by the RC time constant. Therefore, it is an ideal device for high frequency and fast switching, and its operating frequency can up to 100GHz. In addition, MIS (metal-insulator-semiconductor) Schottky diodes can be used to manufacture solar cells or light-emitting diodes.

Fast recovery diode, with a forward voltage drop of 0.8-1.1V, reverse recovery time of 35-85nS, can transform operating state (conductive and cut-off) quickly, increasing the frequency of use of the device and improving the waveform.

Schottky diodes have the advantages of good switching characteristics, short reverse recovery time, large forward current, small size and easy installation. The superfast recovery diode (SRD) is developed on the basis of a fast recovery diode, its reverse recovery time is close to that of the Schottky diode. They can be widely used in switching power supplies, pulse width modulators (PWM), uninterruptible power supplies, AC motor variable frequency speed regulation, high frequency heating, etc., as a high frequency, high current freewheeling diode or a rectifier.